MIMO (Multiple Input, Multiple Output) communication method has been proposed as transmission technology to increase the wireless communication speed on wireless LANs. In MIMO, a plurality of antennas is provided, and different transmission signal are transmitted simultaneously from a plurality of transmission antennas in the same channel by choosing channel or frequency. By this transmission, the overall transmission quantity can be increased without expanding the frequency bandwidth. That is, the transmission signal series can be increased without expanding the frequency band, so that the efficiency of frequency utilization and the wireless transmission speed may be increased.
Further, when performing diversity transmission, a plurality of antennas are provided, an antenna with high receiver gain would have high sensitivity. It also receive higher power via different transmission paths.
Antennas used in MIMO communication methods and diversity transmission methods are described in Japanese Patent Laid-open No. 2007-142878, Japanese Patent Laid-open No. 2007-13643, and in “Study Relating to Reduced Mutual Coupling Between L-shape Loopback Monopole Antenna Elements for Portable Terminals” (Keitai Tanmatsu yo L-ji gata Orikaeshi Monopo-ru Antena no Soshi kan Sougo Ketsugou Teigen ni Kansuru Ichi Kentou), Yongho Kim, Jun Itoh, and Hisashi Morishita, Department of Electrical and Electronic Engineering, National Defense Academy of Japan, IEICE Tech. Rep., announced at Okinawa Univ., Mar. 27, 2008. In Japanese Patent Laid-open No. 2007-142878, a multi-antenna for terminals is described that when a plurality of antenna elements are used in wireless terminal device, the first antenna group is set in a first place, and a second antenna group in a second place perpendicular to the first one, and it proved the influence of mutual coupling of the first and second antennas is reduced.
Further, in Japanese Patent Laid-open No. 2007-13643, an integral-type plate multi-element antenna is described, First and second radiating elements are provided, having feed portions on both sides of the cutout portion of a ground pattern having a cutout portion, so that the electromagnetic interaction between radiating elements is reduced, the degree of coupling between radiating elements is reduced, and the characteristics of a plurality of radiating elements are isolated.
IN “Study Relating to Reduced Mutual Coupling Between L-shape Loopback Monopole Antenna Elements for Portable Terminals” (Keitai Tanmatsu yo L-ji gata Orikaeshi Monopo-ru Antena no Soshi kan Sougo Ketsugou Teigen ni Kansuru Ichi Kentou), Yongho Kim, Jun Itoh, and Hisashi Morishita, Department of Electrical and Electronic Engineering, National Defense Academy of Japan, IEICE Tech. Rep., announced at Okinawa Univ., Mar. 27, 2008, a MIMO communication method antenna is described, a conductive bridge is provided which couples the ground terminal portions of a pair of radiating elements, and reduces the mutual coupling between the radiating elements.
In the case of a terminal antenna of the prior art, when a radiating element of the antenna is brought into proximity with the conducting board (circuit board) on which the radiating element is installed, the radiating element and the conducting board undergo electromagnetic interaction, so that the resonance frequency of the antenna is shifted from the desired frequency, and in addition the reflection coefficient (VSWR, voltage Standing Wave Ratio) rises and the antenna gain falls. For example, in the case of the 2.4 GHz band, the element cannot be brought to within λ/16(≈0.125/16≈7.8125 mm) due to the above problem. In particular, an inverted F-type antenna and L-shape antenna used in portable terminals have a low fractional bandwidth (bandwidth relative to the center frequency) of approximately 6%, so that movement of the resonance frequency should be avoided.
On the other hand, in the case of a wireless LAN card inserted into a laptop computer, it is desirable that the antenna is within the card housing. Similar in portable telephones and other portable data terminals, it is desirable that the antenna and the conduction board (circuit board) on which the antenna is mounted be configured compactly. However, as explained above, a radiating element cannot be brought closer than approximately λ/16 to the conduction board, or impeding a compact design.